Dubbed Gliese 581g, the planet is about 120 trillion miles away, circling a red dwarf star, according to a forthcoming report in The Astrophysical Journal. One face of the world is gravitationally locked to face its star, trapped in perpetual sunlight with the far side in perpetual darkness, says the report. Impossible to determine with present telescopes, life there would enjoy a permanent sunset (or sunrise) in a ring stretching from pole to pole, says study co-leader Steven Vogt of the University of California-Santa Cruz.

Smaller than our sun, the star Gliese 581 has five other planets besides Gliese 581g, which has a "year" of just under 37 days for each orbit of its star. All six planets were discovered through observations of the back-and-forth gravitational wobbles they induce on the star, as were most of the roughly 400 planets that have been detected orbiting nearby stars since 1995. Astronomers have reported other planets in the "habitable" zones of their stars since then, but Gliese 581g looks like the best fit yet for a place where life as we know it would thrive.

"It would be a great planet if you liked sunsets," says MIT planetary scientist Sara Seager, who was not part of the study. "What's exciting is this one turned up so close, and so soon, which means there must be many more."

Based on star surveys and similar discoveries of multiple planet solar systems nearby, Vogt estimates that 20 billion to 40 billion worlds orbit the habitable zones of their stars in our Milky Way galaxy. Vogt argues that life likely has sprung up on at least some of these planets if they have oceans. But Seager cautions that, "any talk of life on Gliese 581g is pure speculation."

Assuming Gliese 581g has an atmosphere, average surface temperatures there would range from -24 to 10 degrees Fahrenheit. High-altitude winds, Vogt says, would moderate hotter temperatures on the sunny side and frigid ones on the dark side. Surface winds, meanwhile, would rarely exceed 40 mph. The planet is about 1.2 to 1.4 times wider than Earth, with slightly stronger gravity.

In the past decade, astronomers have described the atmospheres of large "transit" planets with orbits viewable edge-on from Earth. Gliese 581g is not a transit planet, but NASA's Kepler spacecraft may reveal Earth-like worlds suited for such observations within three years.

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